Doped lasing crystals, such as Neodymium Doped Yttrium Aluminum Garnet (Nd:YAG) crystals lase at their natural response wavelength when energized with light at their pump wavelength. Optically isotropic lasing crystals produce a single polarization of light at the natural response wavelength when pumped, whereas optically anisotropic lasing crystals produce two orthogonally oriented polarizations of light at the natural response wavelength when pumped. The generation of two orthogonally oriented polarizations is known as “birefringence.”
The phenomenon of birefringence associated with doped lasing crystals enables the development of photonic sensors for simultaneous detection of various measurands. These measurands include, among others, acoustic field, depth (i.e., the imposition of a static pressure), multi-axis acceleration, particle velocity sensors, orientation sensors. It is known that the application of force to one face of an isotropic crystal creates a small shift in the lattice of the crystal, and thereby results in the temporary creation of an anisotropic crystal which results in a small change in the wavelength of the polarized light associated with the direction of the force. As the force is applied to the crystal, the unforced direction of the lasing crystal also generates light at the natural response frequency of the crystal. Thus, the crystal simultaneously produces two wavelengths of light in orthogonal polarizations that are nearly the same wavelength. The difference in wavelength can be measured as a beat frequency, which is proportional to the force imposed on the crystal.
FIG. 1 is a schematic representation of a force transducer apparatus taken from Holzapfel et al. (Holzapfel, W.; Neuschaefer-Rube, St.; Kobusch, M., “High-resolution, very broadband force measurements by solid-state laser transducers”, Measurement, vol. 28, pp. 277-291, (2000)). In the Holzapfel apparatus, a small diode laser produces a light at the pump wavelength (808 nm in the case of a Nd:YAG lasing crystal). The pump light is focused onto a first coated optical surface of the lasing crystal. A second coated surface of the lasing crystal then generates 1064 nm response light in two orthogonal polarizations. When a force is imposed on a top surface of the crystal, the vertical polarization wavelength of the response light is shifted slightly from its natural frequency. The polarized wavelengths of the response light are then combined with an optical polarizer and the difference frequency, or beat frequency, between the two polarizations are thereafter detected with a photodiode.
Holzapfel et al. used the apparatus of FIG. 1 to measure the beat frequencies for static and dynamic forces placed on Nd:YAG lasing crystals of various sizes with various loading rates. Holzapfel et al. showed that the change in the beat frequency for both statically and dynamically loaded crystals is linear for at least nine decades of force load. Furthermore, the results of the Holzapfel et al. experiments were consistent regardless of the size of the crystal or the method of loading the crystal. The results of the Holzapfel experiment lead to several observations, including: 1) the dynamic range of a sensor employing this concept is strictly limited by the ability to measure a change in the beat frequency of the polarized light; 2) the materials needed to manufacture such sensors are widely available, low-cost components; 3) accurate measurements can be made from DC to the limit of modern phase or frequency detector circuits; and 4) such sensors can be configured to measure force, acceleration, pressure and orientation.
The disclosure of U.S. Pat. No. 6,693,848 to Ambs et al. builds upon the concepts disclosed by Holzapfel. In the '848 patent, Ambs et al. disclose a hydrophone that is an optically-pumped microchip laser that is powered by light at the pump frequency, and produces different beat frequencies related to a pressure field impinging on the microchip laser. Several microchip laser cavities are placed in a single fiber array using fiber splitters. Each crystal is precisely preloaded to produce beat frequencies in a unique band (analogous to wavelength division multiplexing). The pump laser is located near a dry side receiver inside a seismic exploration ship. The frequency of each sensor is measured by placing a linear polarizer oriented at 45 degrees to one of the polarization axes of the sensor's output laser signal. The signal resulting from the polarizer is the beat frequency. Signals from the microchip lasers are reflected onto the single fiber and are converted to electrical energy by photodiodes. The beat frequency of each sensor is measured by a dry side FM receiver. The Ambs et al. approach is advantageous in comparison with conventional, passive optical fiber hydrophones which rely on intensity and/or phase modulation of a reference laser signal. In contrast to signals in passive optical fiber hydrophones, the frequency-modulated signals generated by the Ambs et al. microchip laser sensors are not affected by mechanical perturbations of the fiber telemetry link. Further, intensity fluctuations which can be problematic in interferometer-based passive techniques have no effect on the frequency modulated signals of the Ambs et al. system since the data is not encoded in the intensity.
There remains a need for laser sensors which provide improvements beyond the concepts disclosed by Holzapfel et al. and Ambs et al. In particular, one limiting factor of the seismic array design disclosed by Ambs et al. is the lack of sufficient optical power to create large, many-element arrays. Additionally, the use of fiber coupled to the laser crystals in the Ambs et al. design is costly and requires relatively high optical power for transmitting light through the fiber. Additionally, frequency division techniques are not scalable to larger arrays. It is therefore desirable to provide compact, low power laser sensors that eliminate the need for coupling fiber to the laser crystals. It is also desirable to provide laser sensors that eliminate the need for frequency division multiplexing and issues involved with preloading the sensors to produce beat frequencies in unique bands. It is further desirable to provide improved monitoring systems employing such laser sensors.